Methods for the acylation of maytansinol

ABSTRACT

Disclosed is a method of preparing an amino acid ester of maytansinol by reacting maytansinol with an N-carboxyanhydride of an amino acid (NCA) in the presence of a drying agent. Also disclosed is an improved method of preparing an amino acid ester of maytansinol in which a nucleophile is added to the reaction mixture after completion of the reaction between maytansinol and an N-carboxyanhydride of an amino acid.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/129,268, filed Sep. 12, 2018, which is a continuation of U.S. patentapplication Ser. No. 15/708,346, filed Sep. 19, 2017 and now U.S. Pat.No. 10,100,062; which is a continuation of U.S. patent application Ser.No. 15/269,163, filed Sep. 19, 2016 and now U.S. Pat. No. 9,796,731;which is a continuation of U.S. patent application Ser. No. 14/660,050,filed Mar. 17, 2015 and now U.S. Pat. No. 9,469,655; which is acontinuation of U.S. patent application Ser. No. 14/037,657, filed Sep.26, 2013 and now U.S. Pat. No. 9,012,629; which claims the benefit ofpriority of U.S. Provisional Patent Application No. 61/705,731, filedSep. 26, 2012; all of which are incorporated herein by reference intheir entireties.

FIELD OF INVENTION

The present invention is an improved process for preparing intermediatesin the synthesis of maytansinoids and antibody conjugates thereof.

BACKGROUND OF THE INVENTION

Maytansinoids are highly cytotoxic compounds, including maytansinol andC-3 esters of maytansinol (U.S. Pat. No. 4,151,042), as shown below:

The naturally occurring and synthetic C-3 esters of maytansinol can beclassified into two groups: (a) Maytansine (2) and its analogs (e.g.,DM1 and DM4), which are C-3 esters with N-methyl-L-alanine orderivatives of N-methyl-L-alanine (U.S. Pat. Nos. 4,137,230; 4,260,608;5,208,020; and Chem. Pharm. Bull. 12:3441 (1984)); (b) Ansamitocins,which are C-3 esters with simple carboxylic acids (U.S. Pat. Nos.4,248,870; 4,265,814; 4,308,268; 4,308,269; 4,309,428; 4,317,821;4,322,348; and 4,331,598).

Maytansine (2), its analogs and each of the ansamitocin species are C3esters of maytansinol that can be prepared by esterification ofmaytansinol (1). U.S. Pat. Nos. 7,301,019 and 7,598,375 describe methodsof acylating maytansinol (1), with an N-carboxyanhydride of an aminoacid (NCA, 5), in the presence of a base to form an amino acid ester ofmaytansinol (May-AA, 6) as shown below:

Amino acid esters of maytansinol are valuable intermediates that can becoupled to carboxylic acids to provide maytansinoids. For example,reaction of maytansinol with (4S)-3,4-dimethyl-2, 5-oxazolidinedione(5a) forms N2′-deacetyl-maytansine (6a), which in turn can be coupled to3-(methyldithio)propionic acid (7), usingN-(3-dimethylaminopropyl)-N′-ethylcarbodiimide hydrochloride (EDAC) toform DM1-SMe (8) as shown below:

A significant disadvantage of the acylation reaction that forms aminoacid esters of maytansinol is that it also forms a by-product comprisingan extra N-methyl-alanyl moiety in the C3 side chain, referred to as“extra-NMA” (9). When N2′-deacetyl-maytansine is acylated, extra NMA (9)is also acylated to form extra NMA-DM1-SMe (9a). The structures ofextra-NMA (9) and extra-NMA-DM1-SMe (9a) are shown below:

DM1 (3) can be prepared from DM1-SMe (8) by reduction, which alsoconverts any extra-NMA-DM1-SMe (9a) to extra-NMA-DM1 (10) as shownbelow:

Extra-NMA-DM1 (10) is difficult to remove from DM1 (3) because bothcompounds have similar polarities and give overlapping peaks in the HPLCtrace of purified DM1 (3). DM1 (3) and DM4 (4) are used to prepareantibody conjugates, several of which are currently in clinical trials.

Thus, there is a need to improve the yield and robustness of theprocesses to prepare such maytansinoids and to minimize by-productsformed during reactions used in their preparation.

SUMMARY OF THE INVENTION

It has now been found that addition of a drying agent to the reactionbetween maytansinol and an N-carboxyanhydride of an amino acidsubstantially increases the yield of an amino acid ester of maytansinol,as shown in Examples 1-4. It has also been found that addition of apre-quenching step with a nucleophile following the reaction ofmaytansinol and an N-carboxyanhydride of an amino acid substantiallyreduces formation of undesirable by-products, such as extra-NMA, asshown in Examples 6-8. Based on these discoveries, improved methods ofpreparing an amino acid ester of maytansinol are disclosed herein.

A first embodiment of the invention is a method of preparing an aminoacid ester of maytansinol represented by Formula (I):

wherein R₁ is hydrogen, an optionally substituted C1-C10 alkyl group oran amino acid side chain, provided that, if the amino acid side chainhas a functional group, the functional group is optionally protected;and R₂ is hydrogen or an optionally substituted C1-C10 alkyl group.

The method comprises reacting maytansinol with an N-carboxyanhydride ina reaction mixture additionally comprising a base and a drying agent.The N-carboxyanhydride is represented by the following formula:

All the variables in Formula (II) are as defined in Formula (I).

A second embodiment of the invention is a method of preparing an aminoacid ester of maytaninol represented by Formula (I), comprising: a)reacting maytansinol with an N-carboxyanhydride represented by Formula(II) in a reaction mixture additionally comprising a base; and b)reacting unreacted N-carboxyanhydride from step a) with a nucleophilicreagent. All the variables in Formulas (I) and (II) are as defined inthe first embodiment of the invention.

BRIEF DESCRIPTION OF THE FIGURES

FIGS. 1-2 are schematics showing the acylation ofN2′-deacetyl-maytansine with a carboxylic acid and a condensing agent.

FIGS. 3-4 are schematics showing the acylation ofN2′-deacetyl-maytansine with an activated carboxylic acid.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to methods for preparing an amino acidester represented by Formula (I) from maytansinol and theN-carboxyanhydride represented by Formula (II). The amino acid ester canbe further esterified to prepare maytansinoids such as DM1 and DM4 andthen further elaborated into antibody conjugates of maytansinoid.Preferably, the amino acid ester is represented by Formula (Ia) and theN-carboxyanhydride is represented by Formula (IIa):

The variables in Formulas (Ia) and (IIa) are as described for Formulas(I) and (II).

Preferably for Formulas (I), (II), (Ia) and (IIa), R₁ is the side chainof a naturally occurring amino acid, provided that, if the side chainhas a reactive functional group, the functional group is optionallyprotected; and R₂ is methyl. Alternatively, R₁ is alkyl and R₂ is methylMore preferably, both R₁ and R₂ are methyl.

In the first embodiment of the invention, the method comprises reactingmaytansinol with an N-carboxyanhydride represented by Formula (II) or(IIa) in a reaction mixture additionally comprising a base and a dryingagent.

In a preferred embodiment, the reaction mixture further comprises aLewis acid. Preferred Lewis acids comprise a metal cation.

In another preferred embodiment, maytansinol and the N-carboxyanhydrideare first reacted and the reaction mixture is then contacted with anaqueous solution containing bicarbonate or carbonate or by contactingthe reaction mixture with a metal scavenger. Metal scavengers known inthe art can be used (see, for example, chapter 9 in “The Power ofFunctional Resin in Organic Synthesis” by Aubrey Mendoca, Wiley-VCHVerlag GmbH & Co. KGaA, 2008). Examples of metal scavengers include, butare not limited to, polymer and silica-based metal scavenger (e.g.,QuadraPure™ and QuadraSil™ by Sigma-Aldrich, SiliaMetS® by SiliCycle,Smopex® by Johnson Matthey and Biotage metal scavengers), carbon-basedscavengers (e.g., QuadraPure™ C by Sigma-Aldrich).

In another preferred embodiment, maytansinol and the N-carboxyanhydrideare first reacted and the metal cation from the Lewis acid is thenremoved from the reaction mixture. For example, the metal cation fromthe Lewis acid is removed from the reaction mixture by contacting thereaction mixture with an aqueous solution containing bicarbonate orcarbonate or by contacting the reaction mixture with a metal scavenger.

In the second embodiment, the method comprises: a) reacting maytansinolwith an N-carboxyanhydride represented by formula (II) or (IIa) in areaction mixture additionally comprising a base; b) reacting unreactedN-carboxyanhydride from step a) with a nucleophilic reagent.

In one preferred embodiment, the reaction mixture of step a) furthercomprises a Lewis acid. Preferred Lewis acids comprise a metal cation.

In another preferred embodiment, the reaction mixture after step b) iscontacted with an aqueous solution containing bicarbonate or carbonateor with a metal scavenger.

In another preferred embodiment, the metal cation from the Lewis acid isremoved from the reaction mixture after performing step b), i.e., afterreaction of the nucleophile with the unreacted N-carboxyanhydride. Forexample, the metal cation from the Lewis acid is removed from thereaction mixture by contacting the reaction mixture with an aqueoussolution containing bicarbonate or carbonate or by contacting thereaction mixture with a metal scavenger.

In still another preferred embodiment, the reaction mixture of step a)further comprises a drying agent.

The term “base” refers to a substance that can accept hydrogen ions(protons) or donate a pair of valence electrons. Exemplary bases are nonnucleophilic and non reactive to the N-carboxyanhydride represented byFormula (II). Examples of the suitable bases include a trialkylamine(e.g., diisopropylethylamine, triethylamine, and1,8-Diazabicycloundec-7-ene), a metal alkoxide (e.g., sodiumtert-butoxide and potassium tert-butoxide), an alkyl metal (e.g.,tert-butyllithium, methyl lithium, n-butyl lithium, tert-butyl lithium,lithium di-isopropylamide, pentyl sodium, and 2-phenylisopropyl-potassium), an aryl metal (e.g., phenyl lithium), a metalhydride (e.g., sodium hydride), a metal amide (e.g., sodium amide,potassium amide, lithium diisopropylamide and lithiumtetramethylpiperidide), and a silicon-based amide (e.g., sodiumbis(trimethylsilyl)amide and potassium bis(trimethylsilyl)amide).Preferably, the base is a trialkylamine. More preferably, the base isdiisopropylethylamine.

The term “drying agent” refers to an agent that can remove water from asolution. Examples of a suitable drying agent include, but are notlimited to, molecular sieves, sodium sulfate, calcium sulfate, calciumchloride, and magnesium sulfate. The physical forms of the drying agentsinclude, but are not limited to, granular beads or powders. Preferably,the drying agent is molecular sieve. Alternatively, the drying agent issodium sulfate.

The term “Lewis acid” refers to an acid substance which can employ anelectron lone pair from another molecule in completing the stable groupof one of its own atoms. Exemplary Lewis acids for use in the disclosedmethods include zinc triflate, zinc chloride, magnesium bromide,magnesium triflate, copper triflate, copper (II) bromide, copper (II)chloride, and magnesium chloride. Preferably, the Lewis acid is zinctriflate.

The term “nucleophilic reagent” refers to a reactant that reacts withelectropositive centers in the N-carboxyanhydride represented by Formula(II) to decompose the N-carboxyanhydride. Examples of suitablenucleophilic reagent include water, an alcohol (methanol, ethanol,n-propanol, isopropanol, or tert-butanol) and a primary or secondaryamine (e.g., methylamine, ethylamine, dimethylamine, diethylamine,etc.). Preferably, the nucleophilic reagent is an alcohol.Alternatively, the nucleophilic reagent is water.

Exemplary reaction conditions for preparing the amino acid esters ofmaytansinol represented by Formula (I) are provided below. Specificconditions are provided in Exemplification.

Although equimolar amounts of maytansinol to an N-carboxyanhydride canbe used, more commonly N-carboxyanhydride is used in excess. Exemplarymolar ratios of maytansinol to N-carboxyanhydride range from 1:1 to1:10, more commonly 1:1 to 1:4. \

The Lewis acid is used optionally in the disclosed methods. Whenpresent, it is typically used in excess relative to the maytansinol, forexample, up to a 20 fold excess. More commonly, the molar ratio ofmaytansinol to Lewis acid ranges from 1:5 to 1:8, more preferably about1:7. Lesser amounts of Lewis acid can also be used.

Sufficient amounts of drying agents are used to remove dissolved waterfrom the reaction solvent. The quantity of drying agent is not critical,provided that the reaction solution is rendered substantially anhydrous.The drying agent can be used directly in the reaction vessel or by beingcontained in the vessel by a semi permeable barrier, such as a sinteredglass container.

The time required for the reaction can be easily monitored by oneskilled in the art using techniques including, but not limited to, highpressure liquid chromatography and thin layer chromatography. A typicalreaction is completed after stirring for 24 hours but may be performedat a slower or a faster rate depending on various factors, such asreaction temperature and concentrations of the reactants.

The reaction can be performed between −20° C. through 80° C., preferablybetween −10° C. and 60° C., more preferably between −10° C. to 40° C.,and most preferably between 0° C. and 35° C.

Suitable solvents are readily determined by one of ordinary skill in theart, and include, but are not limited to, polar aprotic solvents such asanhydrous dimethyl formamide, hexanes, ethers (such as tetrahydrofuran,diethyl ether, dimethoxyethane, dioxane), dimethyl sulfoxide (DMSO),dimethylacetamide (DMA), dichloromethane, or a mixture thereof.

If a Lewis acid is present in the reaction mixture, the reaction mixtureafter the reaction of maytansinol and the N-carboxyanhydride ispreferably contacted with an aqueous solution containing bicarbonate orcarbonate or with a metal scavenger. Preferably, the reaction mixture isreacted with the nucleophilic reagent to decompose excessN-carboxyanhydride prior to the reaction mixture being contacted with anaqueous solution containing bicarbonate or carbonate or with a metalscavenger.

If a Lewis acid comprising a metal cation is present in the reactionmixture, the metal cation is preferably removed from the reactionmixture as part of the reaction work-up. Removal of the metal cation canbe accomplished by contacting the reaction mixture with an aqueoussolution containing bicarbonate or carbonate or with a metal scavenger.Preferably, the N-carboxyanhydride is reacted with the nucleophilicreagent prior to removal of the metal cation.

The amount of a nucleophile in step b) can be readily determined by askilled person in the art. Preferably, a sufficient quantity ofnuclophile is used to decompose the unreacted N-carboxyanhydride. Thisis typically an equimolar quantity of nucleophile, however, excessquantities of nucleophile can also be used. A typical reaction iscompleted after stirring 1 hour but may be performed at a slower or afaster rate depending on various factors, such as temperature.

Also, within the scope of the invention is a method of acylating theamino acid ester of maytansinol. The method comprises reacting an aminoacid ester of maytansinol represented by Formula (I) or Formula (Ia)prepared as described above with a carboxylic acid, having the formula“R₃COOH”, in the presence of a condensing agent or with an activatedcarboxylic acid having the formula “R₃COX”, to form a compoundrepresented by one of the following formulas, respectively:

In Formula (III) or (IIIa), R₁ and R₂ are as defined in Formulas (I),(II), (Ia), and (IIa); R₃ is an alkyl group or a substituted alkylgroup; and X in R₃COX is a leaving group. Preferably, X is a halide, analkoxy group, an aryloxy group, an imidazole, —S-phenyl, in which phenylis optionally substituted with nitro or chloride, or —OCOR, in which Ris a linear C1-C10 alkyl group, a branched C1-C10 alkyl group, a cyclicC3-C10 alkyl group, or a C1-C10 alkenyl group. In one embodiment, in theformula “R₃COX” described above, —COX is a reactive ester; for examplean optionally substituted N-succinimide ester. Examples of a reactiveester include, but are not limited to, N-succinimidyl,N-sulfosuccinimidyl, N-phthalimidyl, N-sulfophthalimidyl, 2-nitrophenyl,4-nitrophenyl, 2,4-dinitrophenyl, 3-sulfonyl-4-nitrophenyl and3-carboxy-4-nitrophenyl esters.

Preferably, R₃ is —Y—S—SR₄, Y is C1-C10 alkylene, and R₄ is C1-C10alkyl, aryl, or heteroaryl. In another alternative, Y is —CH₂CH₂— or—CH₂CH₂C(CH₃)₂— and R₄ is methyl.

In another embodiment, R₃ is -L-E; L is

or —(CH₂CH₂O)_(m)CH₂CH₂NHC(═O)CH₂CH₂— or

E is

X′ is a halide; n is 1, 2, 3, 4, 5 or 6; m is 0 or an integer from 1 to20; and q is 0 or 1. Alternatively, L is —(CH₂)_(n)—; and n is asdefined above or n is 5. In another alternative, L is

and n and m are as defined above; or, alternatively, n is 4 and m is 3.

In yet another alternative, R₃ is selected from the following formulas:

The term “condensing agent” is a reagent that reacts with the hydroxylgroup of a carboxylic acid and converts it into a leaving group, whichcan be displaced by an amine. Examples of suitable condensing agentsinclude a carbodiimide (N-(3-dimethylaminopropyl)-N′-ethylcarbodiimidehydrochloride), a uronium, an active ester, a phosphonium,2-alkyl-1-alkylcarbonyl-1,2-dihydroquinoline(2-isobutoxy-1-isobutoxycarbonyl-1,2-dihydroquinoline),2-alkoxy-1˜alkoxycarbonyl-1,2-dihydroquinoline(2-ethoxy-1˜ethoxycarbonyl-1,2-dihydroquinoline), or alkylchloroformate(isobutylchloroformate). Preferably, the condensing agent is acarbodiimide. More preferably,N-(3-dimethylaminopropyl)-N′-ethylcarbodiimide hydrochloride.

The term “leaving group” refers to a group of charged or unchargedmoiety that can readily be displaced by a nucleophile, such as an amine.Such leaving groups are well known in the art and include, but notlimited to, halides, esters, alkoxy, hydroxyl, alkoxy, tosylates,triflates, mesylates, nitriles, azide, an imidazole, carbamate,disulfides, thioesters, thioethers (i.e., —S-phenyl optionallysubstituted) and diazonium compounds. Preferably, the leaving group is ahalide, an alkoxy group, an aryloxy group, an imidazole, —S-phenyloptionally substituted with —NO₂ or Chloro, or —OCOR, in which R is alinear C1-C10 alkyl group, a branched C1-C10 alkyl group, a cyclicC3-C10 alkyl group, or a C1-C10 alkenyl group. In another preferredembodiment, the leaving group is the moiety in a reactive ester (e.g.,—COX) that can be displaced. A reactive ester includes, but is notlimited to N-succinimidyl, N-sulfosuccinimidyl, N-phthalimidyl,N-sulfophthalimidyl, 2-nitrophenyl, 4-nitrophenyl, 2,4-dinitrophenyl,3-sulfonyl-4-nitrophenyl and 3-carboxy-4-nitrophenyl ester.

The invention also includes a method of using a C3 ester of maytansinolto prepare a derivative thereof. The method comprises reacting a C3ester of maytansinol represented by Formula (III) or (IIIa) preparedabove with a reducing agent to form a compound represented by one of thefollowing formulas:

In Formula (IV) and (IVa), R₁ and R₂ are as defined in Formulas (I),(II), (Ia), and (IIa); and Y is as defined in Formula (III) or (IIIa).

The term “reducing agent” is the element or compound in areduction-oxidation reaction that convert a disulfide bond to ahydrosulfide group. Examples of suitable reducing agents includedithiothreitol (DTT), (tris(2-carboxyethyl)phosphine) (TCEP) and NaBH₄.

The compound of formula (III) or (Ma), when R₃ is -L-E, or the compoundof formula (IV) or (IVa) can react with an antibody or a modifiedantibody to form an antibody-maytansinoid conjugate. See for example,U.S. Pat. Nos. 7,521,541, 5,208,020, and 7,811,872. Alternatively, thecompound of formula (IV) or (IVa) can react with a bifunctionalcrosslinker to form a linker compound carrying a reactive group that canreact with an antibody to form an antibody-maytansinoid conjugate. Seefor example, U.S. Pat. No. 6,441,163, US2011/0003969A1 andUS2008/0145374.

“Alkyl” as used herein refers to a linear, branched or cyclic alkyl.

“Linear or branched Alkyl” as used herein refers to a saturated linearor branched-chain monovalent hydrocarbon radical of one to twenty carbonatoms. Examples of alkyl include, but are not limited to, methyl, ethyl,1-propyl, 2-propyl, 1-butyl, 2-methyl-1-propyl, —CH₂CH(CH₃)₂, 2-butyl,2-methyl-2-propyl, 1-pentyl, 2-pentyl, 3-pentyl, 2-methyl-2-butyl,3-methyl-2-butyl, 3-methyl-1-butyl, 2-methyl-1-butyl,—CH₂CH₂CH(CH₃)₂,1-hexyl, 2-hexyl, 3-hexyl, 2-methyl-2-pentyl,3-methyl-2-pentyl, 4-methyl-2-pentyl, 3-methyl-3-pentyl,2-methyl-3-pentyl, 2,3-dimethyl-2-butyl, 3,3-dimethyl-2-butyl, 1-heptyl,1-octyl, and the like. Preferably, the alkyl has one to ten carbonatoms. More preferably, the alkyl has one to four carbon atoms.

“Alkylene” as used herein refers to a linear, branched or cyclicalkylene.

“Linear or branched Alkylene” as used herein refers to a saturatedlinear or branched-chain divalent hydrocarbon radical of one to twentycarbon atoms. Examples of alkyl include, but are not limited to,methylene, ethylene, 1-propylene, 2-propylene, 1-butylene,2-methyl-1-propylene, —CH₂CH(CH₃)₂—, 2-butylene, 2-methyl-2-propylene,1-pentylene, 2-pentylene, 3-pentylene, 2-methyl-2-butylene,3-methyl-2-butylene, 3-methyl-1-butylene, 2-methyl-1-butylene,—CH₂CH₂CH(CH₃)₂—, 1-hexyl, 2-hexylene, 3-hexylene, 2-methyl-2-pentylene,3-methyl-2-pentylene, 4-methyl-2-pentylene, 3-methyl-3-pentylene,2-methyl-3-pentylene, 2,3-dimethyl-2-butylene, 3,3-dimethyl-2-butylene,1-heptylene, 1-octylene, and the like. Preferably, the alkylene has oneto ten carbon atoms. More preferably, the alkylene has one to fourcarbon atoms.

“Linear or branched Alkenyl” refers to linear or branched-chainmonovalent hydrocarbon radical of two to twenty carbon atoms with atleast one site of unsaturation, i.e., a carbon-carbon, double bond,wherein the alkenyl radical includes radicals having “cis” and “trans”orientations, or alternatively, “E” and “Z” orientations. Examplesinclude, but are not limited to, ethylenyl or vinyl (—CH═CH₂), allyl(—CH₂CH═CH₂), and the like. Preferably, the alkenyl has two to tencarbon atoms. More preferably, the alkenyl has two to four carbon atoms.

“Cyclic alkyl” refers to a monovalent saturated carbocyclic ringradical. Preferably, the cyclic alkyl is three to ten memberedmonocyclic ring radical. More preferably, the cyclic alkyl iscyclohexyl.

“Aryl” means a monovalent aromatic hydrocarbon radical of 6-18 carbonatoms derived by the removal of one hydrogen atom from a single carbonatom of a parent aromatic ring system. Aryl includes bicyclic radicalscomprising an aromatic ring fused to a saturated, partially unsaturatedring, or aromatic carbocyclic or heterocyclic ring. Typical aryl groupsinclude, but are not limited to, radicals derived from benzene (phenyl),substituted benzenes (e.g., para-nitrophenyl, ortho-nitrophenyl, anddinitrophenyl), naphthalene, anthracene, indenyl, indanyl,1,2-dihydronapthalene, 1,2,3,4-tetrahydronapthyl, and the like.Preferably, the aryl is optionally substituted phenyl (e.g., phenyl,phenol or protected phenol).

“Heteroaryl” refers to a monovalent aromatic radical of 5- or 6-memberedrings, and includes fused ring systems (at least one of which isaromatic) of 5-18 atoms, containing one or more heteroatomsindependently selected from nitrogen, oxygen, and sulfur. Examples ofheteroaryl groups are pyridinyl (e.g., 2-hydroxypyridinyl), imidazolyl,imidazopyridinyl, pyrimidinyl (e.g., 4-hydroxypyrimidinyl), pyrazolyl,triazolyl, pyrazinyl, tetrazolyl, furyl, thienyl, isoxazolyl, thiazolyl,oxazolyl, isothiazolyl, pyrrolyl, quinolinyl, isoquinolinyl, indolyl,benzimidazolyl, benzofuranyl, cinnolinyl, indazolyl, indolizinyl,phthalazinyl, pyridazinyl, triazinyl, isoindolyl, pteridinyl, purinyl,oxadiazolyl, triazolyl, thiadiazolyl, furazanyl, benzofurazanyl,benzothiophenyl, benzothiazolyl, benzoxazolyl, quinazolinyl,quinoxalinyl, naphthyridinyl, and furopyridinyl.

Suitable substituents for an alkyl group are those which do notsignificantly interfere with the disclosed reactions. Substituents thatdo interfere with the disclosed reactions can be protected according tomethods well known to one of ordinary skill in the art, for example, inT. W. Greene and P. G. M. Wuts “Protective Groups in Organic Synthesis”John Wiley & Sons, Inc., New York 1999. Exemplary substituents includearyl (e.g., phenyl, phenol and protected phenol), heteroaryl (e.g.,indolyl dimidazolyl) halogen, guanidinium [—NH(C═NH)NH₂], —OR¹⁰⁰,NR¹⁰¹R¹⁰², —NO₂, —NR₁₀₁COR¹⁰², —SR¹⁰⁰, a sulfoxide represented by—SOR¹⁰¹, a sulfone represented by —SO₂R¹⁰¹, a sulfate —SO₃ R¹⁰⁰, asulfonate —OSO₃ R¹⁰⁰, a sulfonamide represented by —SO₂NR¹⁰¹R¹⁰², cyano,an azido, —COR¹⁰¹, —OCOR¹⁰¹, —OCONR¹⁰¹R¹⁰²; R¹⁰¹ and R¹⁰² are eachindependently selected from H, linear, branched or cyclic alkyl, alkenylor alkynyl having from 1 to 10 carbon atoms.

The term “halide” refers to —F, —Cl, —Br or —I.

The term “amino acid” refers to naturally occurring amino acids ornon-naturally occurring amino acid represented byNH₂—C(R^(aa′)R^(aa))—C(═O)OH, wherein R^(aa) and R^(aa′) are eachindependently H, an optionally substituted linear, branched or cyclicalkyl, alkenyl or alkynyl having 1 to 10 carbon atoms, aryl, heteroarylor heterocyclyl. The term “amino acid” also refers to the correspondingresidue when one hydrogen atom is removed from the amine and/or carboxyend of the amino acid, such as —NH—C(R^(aa)′R^(aa))—C(═O)O—. Thespecific examples below are to be construed as merely illustrative, andnot limitative of the remainder of the disclosure in any way whatsoever.Without further elaboration, it is believed that one skilled in the artcan, based on the description herein, utilize the present invention toits fullest extent. All publications cited herein are herebyincorporated by reference in their entirety. Further, any mechanismproposed below does not in any way restrict the scope of the claimedinvention.

EXEMPLIFICATION

Materials and Methods

The process parameters given below can be adopted and adapted by skilledpersons to suit their particular needs.

All reactions were performed under an argon atmosphere with magneticstirring. Tetrahydrofuran and dimethyl formamide were purchased asanhydrous solvents from Aldrich. Maytansinol, was produced as described(Widdison et. al. J. Med. Chem. 49: 4392-4408 (2006)). TheN-carboxyanhydride of N-methyl-alanine, (4S)-3,4-dimethyl-2,5-oxazolidinedione was prepared as described (Akssira, M. et. al. J.Marocain de Chimie Heterocyclique 1: 44-47 (2002)). Nuclear magneticresonance (NMR) spectra (¹H 400 MHz, ¹³C 100 MHz) were obtained on aBruker ADVANCE™ series NMR. HPLC/MS data was obtained using a BrukerESQUIRE™ 3000 ion trap mass spectrometer in line with an Agilent 1100series HPLC. HPLC method 1 was used to analyze DM1. HPLC method 2 wasused for all other analyses.

Analytical HPLC method 1:

Water HPLC system with UV detector or equivalent

Column: YMC-Pack ODS-AQ 250×4.6 mm; 5 μm (Part #=AQ12S05-2546WT)

Flow: 1 mL/min (Gradient)

Mobile Phase: A=1 ml of 85% H₃PO₄ in 1 liter water;B=acetonitrile/Tetrahydrofuran 30:70 (v/v) (Note: 0.1% TFA was usedinstead of H3PO4 in the mobile phase A in LC/MS analysis

Gradient table: Time, min Flow % A % B 1 0.0 1.00 62 38 2 25 1.00 62 383 40 1.00 40 60 4 60 1.00 40 60Run time: 60 minutes+Post time: 10 minutesUV detection: 252 nmInjection volume=5 μL of about 1 mg/ml of DM1 in acetonitrileColumn temperature=15° C. (unless otherwise stated)Sample temperature=2-8° C.

Analytical HPLC/MS method 2:

Column: 150×4.6 mm C8, particle size 5 micron, Zorbax P/N 993967-906

Solvents: A deionized water+0.1% TFA

Solvent B: Acetonitrile

Flow rate 1.0 mL/min Temperature: Ambient

Injection volume: 15 μL

Gradient Time % B 0 25 25 50 26 95 30 95 31 25 37 25Data was displayed from 0-25 min in HPLC traces.

Sample preparation for analytical HPLC method 2:

Aliquots (20 μL) of a given mixture were added to acetonitrile (1.5 mL)in an autosampler vial. The vial was capped and shaken then placed in a15° C. autosampler. An injection volume (15 μL) was analyzed for eachHPLC run.

Example 1: Preparation of DM1-SMe with Added 4A Molecular Sieves asDrying Agent

Maytansinol (50.1 mg, 0.0888 mmol), (4S)-3,4-dimethyl-2,5-oxazolidinedione (30.2 mg, 0.233 mmol, 2.6 eq), zinc triflate (133 mg,0.366 mol) and 4A Molecular sieves (0.50 g) pre-dried at 250° C. undervacuum then cooled to ambient temperature, were added to a 10 ml flask.The contents were taken up in anhydrous dimethyl formamide (0.75 mL) towhich was added diisopropylethyl amine (62 μL, 0.357 mmol). The mixturewas stirred at ambient temperature for 24 hr. A sample of the crudemixture was analyzed by HPLC, N^(2′)-deacetyl-maytansine productaccounted for 80% of the total HPLC area. The reaction mixture wasdiluted with 1:1 saturated NaHCO₃:saturated NaCl (1.2 mL) and ethylacetate (3 mL) mixed then filtered with celite, then washed withpotassium phosphate buffer (1 mL, 400 mM, pH 7.5). The organic layer wasdried with anhydrous magnesium sulfate, filtered then evaporated to forma yellow solid. To the solid was added 3-methyldithiopropanoic acid (25mg, 0.16 mmol), N-(3-dimethylaminopropyl)-N′-ethylcarbodiimidehydrochloride (30 mg, 0.16 mmol) and dichloromethane (3 mL). Afterstirring for 2 hr, the mixture was diluted with ethylacetate (8 mL),washed with 1.0 M, pH 6.5 potassium phosphate buffer (2 mL) and theaqueous solution was extracted with ethyl acetate (2×8 mL). The organiclayers were combined, dried over anhydrous magnesium sulfate,concentrated and purified by silica chromatography 95:5dichloromethane:methanol to afford 51 mg (70%) of DM1-SMe.

Example 2: 10× Scale Up of Example 1

The reaction in Example 1 was run on a 10 fold larger scale giving 490mg (68%) of DM1-SMe.

Example 3: Preparation of DM1-SMe without Added Drying Agent

Maytansinol (1.0 g, 1.77 mmol) was dissolved in anhydrous dimethylformamide (15 mL) in a 25 mL flask which was cooled in an ice/waterbath. After 2 min diisopropylethyl amine (DIPEA, 0.92 g 7.07 mmol) andzinc triflate (3.8 g, 10.6 mmol) were added with magnetic stirring, then(4S)-3,4-dimethyl-2, 5-oxazolidinedione (0.913 g, 7.07 mmol) was quicklyadded and the mixture was stirred for 24 hr. A sample of the crudemixture was analyzed by HPLC, N^(2′)-deacetyl-maytansine productaccounted for 65% of the total HPLC area. The reaction mixture wasdiluted with 1:1 saturated NaHCO₃:saturated NaCl (25 mL) and ethylacetate (40 mL), mixed then filtered with celite, and washed withsaturated NaCl. The organic layer was dried with anhydrous sodiumsulfate, filtered then evaporated. Residue was taken up indichloromethane (30 mL) to which 3-methyldithiopropanic acid (1.1 g, 7.0mmol) and N-(3-dimethylaminopropyl)-N′-ethylcarbodiimide hydrochloride(1.34 g, 7.0 mmol) were quickly added and the reaction was stirred underargon at ambient temperature for 2 hours. The mixture was diluted withethyl acetate (30 mL), washed with 1.0 M potassium phosphate buffer (30mL), pH 6.5, and the aqueous solution was extracted with ethyl acetate(2×40 mL). The organic layers were combined, dried over anhydrous sodiumsulfate, concentrated and purified by silica chromatography 95:5dichloromethane:methanol to afford 698 mg (50%) of DM1-SMe.

Example 4: Repeat of Example 3

The reaction in Example 3 was repeated on the same scale giving 735 mg(53%) of DM1-SMe.

Example 5: Crude N2′-Deacetyl-Maytansine Stock Solution

Maytansinol (0.5 g, 0.89 mmol) was dissolved in anhydrous dimethylformamide (7 mL) in a 25 mL flask which was cooled in an ice/water bath.After 2 min diisopropyl ethyl amine (0.5 g, 3.5 mmol) and zinc triflate(1.9 g, 5.3 mmol) were added with magnetic stirring, then(4S)-3,4-dimethyl-2, 5-oxazolidinedione (4.52 g, 3.5 mmol) was quicklyadded and the mixture was stirred for 24 hr. Aliquots (0.5 mL each) ofthis stock solution were used in the following experiments thus eachaliquot was generated from approximately 0.13 mmol of maytansinol.

Example 6: N^(2′)-Deacetyl-Maytansine Extraction Followed by Coupling toPropionic Acid (Control)

N^(2′)-deacetyl-maytansine stock solution (0.50 mL) was added to a 6 mLcapacity vial containing ethyl acetate (1.5 mL) and 1:1 saturatedNaCl:NaHCO₃ (0.75 mL), quickly capped and mixed. The organic layer wasretained and dried over anhydrous Na₂SO₄ (120 mg). Organic layer (1.0mL) was taken and propionic acid (20.0 μL, 0.27 mmol). The solution wasthen transferred to a vial containingN-(3-dimethylaminopropyl)-N′-ethylcarbodiimide hydrochloride (40 mg,0.209 mmol). The reaction was allowed to progress for 2.5 hr after whichit was analyzed by HPLC.

The following by-product was also produced from May-NMA2, a by-productin the preceding reaction, as shown below:

The ratio of HPLC percent areas for 17:16 was 3.0:71.7. MS of 16 (M+H+)706 (M+Na⁺) 728; MS of 17 (M+Na⁺) 813.

Example 7: The Experiment of Example 6 was Repeated

The ratio of HPLC percent areas for 17:16 was 3.0:70.9.

Example 8: N²′-Deacetyl-Maytansine Extraction Followed by a MethanolPre-Quench then Coupling to Propionic Acid (Pre-Quench to Destroy Excess5a)

N^(2′)-deacetyl-maytansine stock solution (0.50 mL) was added to a 6 mLcapacity vial to which methanol (75 μL, 1.8 mmol) was added and the vialwas capped and contents magnetically stirred for 1 hour. Ethyl acetate(1.5 mL) and 1:1 saturated NaCl:NaHCO₃ (0.75 mL) were then added and thevial was capped and mixed. The organic layer was retained and dried overanhydrous Na₂SO₄ (120 mg). Organic layer (1.0 mL) was taken andpropionic acid (20.0 μL, 0.27 mmol) was added. The solution was thentransferred to a vial containingN-(3-dimethylaminopropyl)-N′-ethylcarbodiimide hydrochloride (40 mg,0.209 mmol). The reaction was allowed to progress for 2.5 hr after whichit was analyzed by HPLC. The HPLC peak for 17 was barely detectable,integration was not possible. This reaction was repeated and again 17was barely detectable, integration was not possible. Thus, thepre-quenching method produces less undesirable compounds 15 and 17.

Example 9. Synthesis of Extra-NMA-DM1-SMe (9a)

Maytansinol (1.2 mg, 2.1 mmol) was weighed into a 50 mL flask anddissolved in a mixture of dimethylformamide (12 mL) and tetrahydrofuran(6 mL). The flask was cooled in an ice/water bath. After 5 mindiisopropyl ethyl amine (1.5 mL, 8.5 mmol), zinctrifluoromethanesufonate (4.5 g, 12.6 mmol), and 2,5-oxazolidinedione,3,4-dimethyl (4S) (1.1 g, 8.5 mmol) were sequentially added. Afterstirring for 17 hours the reaction was extracted with 1:1 saturatedaqueous NaCl:saturated aqueous NaHCO₃ (14 mL) and Ethyl acetate (100mL). The organic layer was retained and dried over anhydrous Na₂SO₄.Drying agent was removed and approximately ⅔rds of the solvent wasremoved by rotary evaporation under vacuum. ThenN-methyl-N-[(2-methyldithio)-1-oxopropyl]-L-alanine (1.0 g, 4.2 mmol)was added followed by N-(3-dimethylaminopropyl)-N-ethylcarbodiimidehydrochloride (0.889 g, 4.6 mmol). Methylene chloride (10 mL) was addedto dissolve the mixture. After 4 hours the reaction was extracted withmethylene chloride (70 mL) and 1:4 saturated aqueous NaCl:saturatedaqueous NaHCO₃ (20 mL). The organic layer was retained and dried overanhydrous Na₂SO₄. Solvent was removed by rotary evaporation undervacuum. The resulting thick oil was dissolved in acetonitrile (3 mL) andapproximately ½ of the material was purified by HPLC on a waterssymmetry shield C8 column (19×150 mm micron, 5 micron particle size).The column was eluted with deionized water containing 0.2% formic acidwith an acetonitrile gradient (30%-60% acetonitrile over 18 min). Thecolumn was flushed with 95% acetonitrile for 5 min and thenre-equilibrated with 30% acetonitrile for 6 min between runs. Injectionvolumes ranged between 100-800 uL. Unreacted maytansinol eluted at 8.5min, an undesired isomer of Extra-NMA-DM1-SMe eluted at 13.8 min and thedesired isomer of Extra-NMA-DM1-SMe eluted at 15.1 min. Fractions ofdesired product from several runs were combined and solvent was removedby rotary evaporation under vacuum. The residue was taken up in aminimum volume of ethyl acetate and a minor impurity was removed by HPLCon a Kromasil cyano column (250 mm×21 mm, 10 micron particle size). Thecolumn was run with an isocratic mobile phase of 67:9:24hexanes:2-propanol:ethyl acetate at 21 mL/min. The desired producteluted at 22.6 min while the impurity eluted at 12.6 min. Productfractions from several runs were combined and solvent was removed byrotary evaporation under vacuum to provide 95 mg of product (10% yield).1H NMR (400 MHz, CDCl₃-d) δ=7.26, 6.81 (d, J=1.6 Hz, 1H), 6.67 (d,J=11.1 Hz, 1H), 6.56 (d, J=1.6 Hz, 1H), 6.42 (dd, J=11.4, 15.2 Hz, 1H),6.30 (s, 1H), 5.67 (dd, J=9.1, 15.2 Hz, 1H), 5.52-5.40 (m, 1H), 5.27 (d,J=7.1 Hz, 1H), 4.85-4.69 (m, 1H), 4.26 (t, J=10.9 Hz, 1H), 3.96 (s, 3H),3.7 (bs, 1), 3.57 (d, J=12.6 Hz, 1H), 3.48 (d, J=8.8 Hz, 1H), 3.34 (s,3H), 3.23 (s, 3H), 3.10 (d, J=12.6 Hz, 1H), 3.03-2.90 (m, 3H), 2.87 (s,3H), 2.82-2.64 (m, 5H), 2.63-2.50 (m, 1H), 2.45-2.30 (m, 3H), 2.15 (d,J=14.1 Hz, 1H), 1.62 (s, 3H), 1.57 (d, J=13.6 Hz, 1H), 1.45 (d, J=6.3Hz, 1H), 1.29 (d, J=7.1 Hz, 3H), 1.26 (d, J=6.3 Hz, 4H), 1.18 (d, J=6.3Hz, 3H), 0.79 (s, 3H) 13C NMR (CDCl3, 100 MHz) δ 170.86, 170.50, 170.35,168.69, 156.19, 152.35, 142.2, 140.90, 139.29, 133.27, 128.05, 125.1,122.07, 119.15, 113.31, 88.72, 80.96, 78.51, 74.23, 66.19, 60.66, 60.13,56.81, 56.71, 54.97, 47.90, 46.72, 38.99, 36.41, 35.68, 33.19, 32.54,30.90, 30.02, 23.01, 15.62, 14.75, 14.59, 13.54, 12.35. HRMS calc. forC₄₀H₅₇C1N₄O₁₁S₂ (M+Na⁺) m/z=891.3052; found 891.3049.

Example 10 Synthesis of Extra-NMA-DM1 (10)

N^(2′)-Deacetyl-N^(2′)-(3-methyldithio-1-oxopropyl-N-methyl-L-alanyl)-maytansine(95 mg, 0.109 mmol) was dissolved in 2:1 dimethoxyethane:100 mMpotassium phosphate buffer pH 7.5 to which dithiothreitol (110 mg, mmol)was added. After 2 hours the solution was extracted with a mixture ethylacetate:methylene chloride=2:1 (5 mL) and saturated aqueous NaCl (1 mL).The organic layer was retained and dried over anhydrous Na2SO4. Thedrying agent was removed by vacuum filtration and solvent was removed byrotary evaporation under vacuum. The residue was taken up in a minimumvolume of 1:1 ethyl acetate:methylene chloride and purified by HPLC on aKromasil cyano column (250 mm×21 mm, 10 micron particle size). Thecolumn was run with an isocratic mobile phase of 64:19:17hexanes:2-propanol:ethyl acetate at 21 mL/min. Desired product eluted at16 min. Fractions of product from several runs were combined end solventwas removed by rotary evaporation to provide 62 mg of product (69%yield). ¹H NMR (400 MHz, CDCl₃) δ 6.81 (d, J=1.6 Hz, 1H), 6.67 (d,J=11.1 Hz, 1H), 6.58 (d, J=1.6 Hz, 1H), 6.43 (dd, J=15.3 Hz, 11.1 Hz,1H), 6.26 (s, 1H), 5.67 (dd, J=15.3 Hz, 9.0 Hz, 1H), 5.47 (q, J=6.6 Hz,1H), 5.28-5.22 (m, J=6.7 Hz, 1H), 4.81 (dd, J=12.0 Hz, 2.9 Hz, 1H), 4.26(t, J=10.5 Hz, 1H), 3.96 (s, 3H), 3.59 (d, J=12.7 Hz, 1H), 3.49 (d,J=9.0 Hz, 1H), 3.41 (bs, 1H), 3.36 (s, 3H), 3.24 (s, 3H), 3.11 (d,J=12.7 Hz, 1H), 2.98 (d, J=9.6 Hz, 1H), 2.85 (s, 3H), 2.84-2.80 (m, 1H),2.79 (s, 3H), 2.76 (s, 1H), 2.68-2.61 (m, 2H), 2.58 (d, J=12.1 Hz, 1H),2.17 (dd, J=14.3 Hz, J=2.8 Hz, 1H), 1.71 (t, J=8.4 Hz, 1H), 1.64 (s,3H), 1.62-1.59 (m, 1H), 1.49-1.40 (m, 1H), 1.31 (d, J=6.9 Hz, 3H), 1.29(d, J=6.4 Hz, 3H), 1.27-1.23 (m, 1H), 1.20 (d, J=6.7 Hz, 3H), 0.81 (s,3H). 13C NMR (CDCl3, 100 MHz) δ 170.37, 170.30, 170.25, 168.53, 156.07,152.16, 142.31, 140.74, 139.16, 133.12, 127.09, 125.32, 121.92, 119.92,113.15, 88.57, 80.83, 78.37, 74.08, 67.01, 59.97, 58.66, 56.56, 53.54,49.17, 46.58, 38.86, 37.33, 36.25, 35.53, 32.39, 30.81, 29.80, 21.02,19.87, 15.47, 14.80, 13.4, 12.22. HRMS calc. for C₃₉H₅₅C1N₄O₁₁S (M+Na⁺)m/z=845.3174; found 845.3166.

What is claimed is:
 1. A method of preparing a compound represented bythe following formula:

wherein R₁ is methyl; and R₂ is methyl, the method comprising: reactingmaytansinol with an N-carboxyanhydride in a reaction mixtureadditionally comprising a base, a drying agent and a Lewis acid, whereinthe molar ratio of maytansinol to N-carboxyanhydride ranges from 1:1 to1:10, and the molar ratio of maytansinol to the Lewis acid ranges from1:5 to 1:8, and wherein the N-carboxyanhydride is represented by thefollowing formula:

thereby forming the compound of Formula (I).
 2. The method of claim 1,wherein the compound of Formula (I) is represented by the followingformula:

and the N-carboxyanhydride is represented by the following formula:


3. The method of claim 1, wherein the Lewis acid is selected from thegroup consisting of zinc triflate, zinc chloride, magnesium bromide,magnesium triflate, copper triflate, copper (II) bromide, copper (II)chloride, and magnesium chloride.
 4. The method of claim 1, furthercomprising contacting the reaction mixture after the reaction ofmaytansinol and the N-carboxyanhydride with an aqueous solutioncontaining bicarbonate or carbonate or contacting the reaction mixturewith a metal scavenger.
 5. A method of preparing a compound representedby the following formula:

wherein R₁ is methyl; and R₂ is methyl, the method comprising: a)reacting maytansinol with an N-carboxyanhydride in a reaction mixtureadditionally comprising a base and a Lewis acid, wherein the molar ratioof maytansinol to N-carboxyanhydride ranges from 1:1 to 1:10, and themolar ratio of maytansinol to Lewis acid ranges from 1:5 to 1:8, andwherein the N-carboxyanhydride is represented by the following formula:

thereby forming the compound of Formula (I); b) reacting unreactedN-carboxyanhydride from the reaction mixture in step a) with anucleophilic reagent.
 6. The method of claim 5, wherein the compound ofFormula (I) is represented by the following formula:

and the N-carboxyanhydride is represented by the following formula:


7. The method of claim 6, wherein the Lewis acid is selected from thegroup consisting of zinc triflate, zinc chloride, magnesium bromide,magnesium triflate, copper triflate, copper (II) bromide copper (II)chloride, and magnesium chloride.
 8. The method of claim 5, wherein thereaction mixture in step a) further comprises a drying agent.
 9. Themethod of claim 5, wherein the nucleophilic reagent is water or analcohol.
 10. The method of claim 5, wherein the method further comprisescontacting the reaction mixture after step b) with an aqueous solutioncontaining bicarbonate or carbonate or contacting the reaction mixturewith a metal scavenger.
 11. The method of claim 1, further comprisingthe step of reacting the compound of formula (I) with a carboxylic acidhaving the formula R₃COOH in the presence of a condensing agent or withan activated carboxylic acid having the formula R₃COX, to form acompound represented by the following formula:

wherein R₃ is an alkyl group or a substituted alkyl group, and X is aleaving group.
 12. The method of claim 11, wherein X is a halide, analkoxy group, an aryloxy group, an imidazole, —S-phenyl optionallysubstituted with nitro or chloride, or —OCOR, in which R is a linearC1-C10 alkyl group, a branched C1-C10 alkyl group, a cyclic C3-C10 alkylgroup, or a C2-C10 alkenyl group.
 13. The method of claim 11, wherein R₃is —Y—S—SR₄, in which Y is C1-C10 alkylene and R₄ is C1-C10 alkyl, aryl,or heteroaryl.
 14. The method of claim 13, further comprising reactingthe compound of formula (III) with a reducing agent to form a compoundrepresented by the following formula:


15. The method of 11, wherein R₃ is -L-E; L is

or —(CH₂CH₂O)_(m)CH₂CH₂NHC(═O)CH₂CH₂— or

E is

X′ is a halide; n is 2, 3, 4, 5 or 6; m is 0 or an integer from 1 to 20;and q is 0 or
 1. 16. The method of claim 11, wherein the condensingagent is a carbodiimide, a uronium, an active ester, a phosphonium,2-alkyl-1-alkylcarbonyl-1,2-dihydroquinoline,2-alkoxy-1-alkoxycarbonyl-1,2-dihydroquinoline, or alkylchloroformate.17. The method of claim 1, wherein the base is a trialkylamine, a metalalkoxide, an alkyl metal, an aryl metal, a metal hydride, a metal amide,or a silicon-based amide.
 18. The method of claim 1, wherein the dryingagent is a molecular sieve, sodium sulfate, calcium sulfate, calciumchloride, or magnesium sulfate.
 19. The method of claim 1, wherein themolar ratio of maytansinol to N-carboxyanhydride is in the range of 1:1to 1:4.
 20. The method of claim 1, wherein the molar ratio ofmaytansinol to Lewis acid is 1:7.